The ability of metamaterials to create artificial electromagnetic properties absent in nature has initiated intense research efforts for applications in frequency selective surfaces, sub-diffraction imaging, cloaking, and etc. See S. Linden et al., “Magnetic response of metamaterials at 100 terahertz,” Science 306, 1351 (2004); X. Zhang, and Z. Liu, “Superlenses to overcome the diffraction limit,” Nature Materials 7, 435 (2008); and J. Valentine et al., “An optical cloak made of dielectrics,” Nature Materials 8, 568 (2009). The development of tunable metamaterials, which allow for real-time tuning of the electromagnetic response, is emerging as an important sub-topic in this field. Tunable metamaterials have the potential to become the building blocks of chip-based active optical devices, and as optical switches, modulators, and phase shifters. A typical way to make such tunable metamaterials is to integrate a natural reconfigurable material in the metamaterial structure and apply an external stimulus to achieve tuning. For example, tunable metamaterials have been demonstrated using electrical reorientation in liquid crystals and thermally/electrically induced insulator-to-metal phase transition in vanadium dioxide (VO2). See D. H. Werner et al., “Liquid crystal clad near-infrared metamaterials with tunable negative-zero-positive refractive indices,” Optics Express 15, 3342 (2007); M. J. Dicken et al., “Frequency tunable near-infrared metamaterials based on VO2 phase transition,” Optics Express 17, 18330 (2009); T. Driscoll et al., “Dynamic tuning of an infrared hybrid-metamaterial resonance using vanadium dioxide,” Applied Physics Letter 93, 024101 (2008); and T. Driscoll et al., “Memory Metamaterials,” Science 325, 1518 (2009).
Recently, active terahertz metamaterials based on variants of split-ring resonators (SRRs) on a doped gallium arsenide (GaAs) substrate have been realized by dynamically changing the carrier concentration of the underlying semiconductor using an electric bias voltage, which effectively tunes the strength of the resonance, producing an amplitude modulation effect or a phase modulation. This amplitude modulation is a result of “shunting” due to the presence of carriers in the doped substrate. See H.-T. Chen et al., “Active terahertz metamaterial devices,” Nature 444, 597 (2006); H.-T. Chen et al., “A metamaterial solid-state terahertz phase modulator,” Nature Photonics 3, 148 (2009); and U.S. Pat. No. 7,826,504 to Chen et al. However, all of these references disclose electrically tunable metamaterials at only terahertz frequencies (i.e., 0.1-3 THz).
Therefore, a need remains for a metamaterial that is tunable in a higher (e.g., infrared) spectral range.